1. Field of the Invention
The present invention relates to a fire alarm system of the light scattering type for detecting an occurrence of a fire from the light scattered by smoke arising from the fire. More specifically, the present invention relates to a fire alarm system which can perform appropriate detection of a fire depending on the type of smoke, according to the relationship between the type of smoke and the scattering angle as well as the degree of polarization of the scattered light. Especially, the present invention relates to a fire alarm system which uses a plane-polarized light source for emitting the light polarized in a predetermined direction so as to achieve accurate and reliable detection of a fire.
2. Description of the Related Art
FIG. 10 illustrates a conventional fire alarm system of the light scattering type, in which a light emitting device 102 such as a light emitting diode is disposed in such a manner that the light emitting device 102 is directed to the center portion X of a smoke detection chamber (smoke detection space). A light receiving device 104 such as a photodiode is disposed in such a manner chat the optical axis of the light receiving device 104 and the optical axis of the light emitting device 102 cross each other at a predetermined angle .theta.. The smoke detection space is always illuminated with the light emitted by the light emitting device 102 which has the directivity in the direction along its optical axis. If a fire occurs and smoke enters the smoke detection space, the light will be scattered by the smoke in the smoke detection space, and the scattered light will be detected by the light receiving device 104 via a converging lens (not shown).
When there is no fire in a normal situation, there is no smoke in the smoke detection space, and thus the intensity of the scattered light detected by the light receiving device 104 is low. On the other hand, if a fire occurs and smoke enters the smoke detection space, the intensity of the scattered light detected by the light receiving device 104 becomes high. There is a correlation between the density of smoke and the intensity of the scattered light which is incident on the light receiving device 104. Therefore, if the output level of the light receiving device 104 exceeds a predetermined threshold level, it is possible to conclude that there is a fire occurring.
However, in conventional fire alarm systems of the type described above, no decision on the smoke type is made, and the occurrence of a fire is detected merely from the density of smoke 106 in the smoke detection space. Therefore, such a conventional fire alarm system has a disadvantage that it cannot perform appropriated detection of a fire depending on the type of smoke.
The color of smoke and the diameters of smoke particles actually vary depending on the material on fire, such as plastic and wood. As a result, even in the case where there is no difference in the density of the smoke 106 in the smoke detection space, the difference in the intensity of the scattered light received by the light receiving device 104 can vary depending on the type of a material which is on fire. Therefore, if the occurrence of a fire is judged based on a constant threshold level neglecting the smoke type, a fire may be misdetected when there is no fire in reality, or otherwise a delay in the fire detection may occur. For example, if a room is filled with smoke of cigarettes, misdetection of a fire may occur when there is no fire in reality. In the case where oil is on fire, the intensity of the light scattered by the black smoke generated during the fire of oil is so low than the fire can be detected only after the fire has been expanded in a certain degree, and thus the fire detection will be delayed.
Some techniques have been proposed to try to solve the above problems. For example, in the technique disclosed in Japanese Patent Application Laid-Open No. 2-213997(1990), nonpolarized light is emitted by a light source, and the components of the scattered light polarized in two directions perpendicular to each other are separately detected. In this technique, the decision of the occurrence of a fire will be made when the ratio between the two components of the light comes in a certain predetermined range.
However, this technique neglects the fact that smoke is a mixture of a large number of particles having various diameters, and the fire detection is done by assuming all smoke particles have the same unique size. As a result, a detection error occurs for actual smoke. Furthermore, this technique uses a light source which emits nonpolarized light, and thus the polarization plane of the light source is not taken at all into consideration. As a result, a reduction occurs in the signal-to-noise ratio of the light received by the light receiving device for components of both polarization directions, and thus the output ratio actually obtained at the light receiving device 104 is not large enough for practical usage. In another technique disclosed in Japanese Patent Application Laid-Open No. 5-128381(1993), it is tried to improve the detection reliability by taking into account the smoke. In this technique disclosed in Japanese Patent Application Laid-Open No. 5-128381(1993), the intensities of the components of the light polarized in different directions are determined, and the degree of polarization is calculated from these intensities. Then, the type of smoke is determined from the result of the calculated degree of polarization. The judgement of occurrence of a fire is made by comparing the light intensity with a preset threshold value depending on the type of smoke. Even in this technique, as in the previous technique described above, the signal-to-noise ratio of the received light is low because this technique also uses a light source which emits nonpolarized light. The output ratio between the case where a fires occurs and the case where no fire occurs is about 2.times.10.sup.-1 :4.times.10.sup.-1, which is not large enough for a practical application.